Functional importance of plumage characteristics (design and costs) as intraspecific signals in Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha)

Background/Question/Methods

Badges of status like bird plumage colors are part of animal communication and mediate interactions between different phenotypes. Honesty of avian plumage badges is thought to be maintained by selective pressures related to life history costs including social punishment.  We tested this hypothesis during two breeding seasons (summers 2008-2009) in a population of Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha; MWCS) near the Rocky Mountain Biological Laboratory, Gothic, Colorado. Both sexes of this species possess a black and white striped crown that has been shown to facilitate intraspecific interactions. To test if honesty of crown whiteness in males is maintained by social punishment, we examined phenotypic variation and associated behaviors. We predicted that crown whiteness was under sexual selection as a dimorphic trait between sexes, and that males with whiter crowns would gain reproductive benefit (measured as number of fledglings).  In addition we predicted that behavioral probing and social aggression along with coriticosterone levels would indicate costs associated with crown whiteness.  MWCS were measured, banded, and paint was used to experimentally enlarge or reduce the proportion of males' crown whiteness.  A series of decoy challenge / call playbacks was used to measure social punishment behavior.  

Results/Conclusions

Males had a significantly greater proportion of crown whiteness than females (t=3.354, df=491, p=0.001). There was no significant difference in number of fledglings for crown manipulated males.  MWCS males received simulated territorial intrusions by a mounted decoy (each twice to the same decoy, both ‘white-enhanced’ and ‘white-diminished’ crown treatments; presented in random order on different days). Males responded with significantly increased alarm rates when presented with white-enhanced decoys (Rates of Chink-calls 2008 [Wald Chi-square=7.233, df=1, p=0.007], Rates of Chink-calls 2009 [Wald Chi-square=7.733, df=1, p=0.005], and body puffs 2009 (Wald Chi-square=4.857, df=1, p=0.028).  MWCS males representing the top 25% of whitest crowns within the population had a significantly lower change in corticosterone response than the rest of the population following a standard stress series test (t = -2.654, df = 30.63, p-value = 0.01250).  Thus birds with whiter crowns draw consistently stronger alarm responses from conspecific territory holders supporting the social punishment hypothesis, yet they are better adapted to limit stress response and maintain breeding activity during their short breeding season.  Social tradeoffs are therefore at least partially responsible for the honesty of crown whiteness as a signal of status in male MWCS and may be influenced by sexual selection.